1. Field of the Invention
The invention relates to a process of producing silicon bodies for solar cells and somewhat more particularly to a process of producing plate- or tape-shaped silicon crystal bodies having crystalline pillar-like structures therein, which are equivalent to crystalline columnar structures, and which bodies are useful for further processing into large-surface solar cells without melting of the base material which forms such silicon bodies.
2. Prior Art
In producing solar cells out of silicon, it is desirable to utilize as economical a silicon material as possible because the property requirements of such components, relative to crystal quality, are not as great as those for semiconductor components used in integrated circuits.
Accordingly, it is desirable to find a means of producing silicon crystal bodies in a relatively simple and inexpensive manner to the extent possible and without excessive material loss. Further, such a process should eliminate or minimize expensive work steps, such as for example, sawing of a silicon rod, which is typically produced in accordance with conventional crystal growth techniques, into crystal disks or plates, as well as lapping and/or polishing of such disks.
German Auslegenschrift (hereinafter DE-AS) 25 08 803 suggests that disk- or plate-shaped silicon crystal bodies having monocrystalline columnar structures therein are well suited for fabrication into solar cells having an efficiency greater than about 10%. The process for producing such bodies described in the above-referenced DE-AS comprise pouring a silicon melt into a suitable mold and allowing plate-shaped silicon bodies to solidify. Such bodies have monocrystalline columnar structures extending in a crystallographic privileged direction. From disks sawed-out of such bodies, which previously were of a maximum size of about 120.times.110 cm.sup.2, solar cells can be fabricated which have an efficiency which fluctuates between about 8.2% at a cell edge, to about 10.5% at the cell center and thus closely approximates the efficiency of solar cells fabricated out of crystalline silicon (which exhibit an efficiency of about 12 to 14%). The starting material for this mold-pouring process is pre-purified polycrystalline silicon, which is melted in a crucible composed of quartz glass under vacuum or protective gas. In the production of plate-shaped bodies, the silicon melt is poured into suitably shaped and chilled casting dyes or molds which are composed of graphite. In order to avoid reaction between the molten silicon and the mold, it, the mold, is cooled to a substantial degree. This cooling also generates a temperature gradient in the silicon, which causes crystallization and columnar structure formation to begin and continue until solidification of the plate-shaped silicon body is attained. In producing solar cells, disks having a thickness of about 500 .mu.m are sawed-out of such plates with diamond saws conventional in silicon and semiconductor technology.
With the foregoing process, the sawing step required for dividing the plates into disks cannot be eliminated. Further, the size of the plates are determined by the size of the mold necessary for their production.
Another method for producing economical silicon material is set forth in Electronics, page 108, Apr. 4, 1974, whereby a polysilicon tape-like body which is at least one meter in length is produced by pouring molten silicon onto a cooled moving support member composed of molybdenum or onto a travelling endless belt coated with a silicon nitride layer, somewhat similar in principle to a conventional conveyor line. However, the resultant silicon material exhibits no crystalline columnar structures and when fabricated into solar cells, displays an efficiency in the range less than about 5%.